Christina Andica

SyMRI of the Brain: Rapid Quantification of Relaxation Rates and Proton Density, With Synthetic MRI, Automatic Brain Segmentation, and Myelin Measurement

Abstract Conventional magnetic resonance images are usually evaluated using the image signal contrast between tissues and not based on their absolute signal intensities. Quantification of tissue parameters, such as relaxation rates and proton density, would provide an absolute scale; however, these methods have mainly been performed in a research setting. The development of rapid quantification, with scan times in the order of 6 minutes for full head coverage, has provided the prerequisites for clinical use. The aim of this review article was to introduce a specific quantification method and synthesis of contrast-weighted images based on the acquired absolute values, and to present automatic segmentation of brain tissues and measurement of myelin based on the quantitative values, along with application of these techniques to various brain diseases. The entire technique is referred to as “SyMRI” in this review. SyMRI has shown promising results in previous studies when used for multiple sclerosis, brain metastases, Sturge-Weber syndrome, idiopathic normal pressure hydrocephalus, meningitis, and postmortem imaging.

Dural Enhancement in a Patient with Sturge-Weber Syndrome Revealed by Double Inversion Recovery Contrast Using Synthetic MRI

1Department of Radiology, Juntendo University School of Medicine, 2-1-1 Hongo, Bunkyo-ku, Tokyo 113-8421, Japan 2Department of Radiology, Graduate School of Medicine, The University of Tokyo 3Department of Radiological Sciences, Graduate School of Human Health Sciences, Tokyo Metropolitan University 4Department of Neurosurgery, Juntendo University School of Medicine (Received June 10, 2015; Accepted July 20, 2015; published online November 6, 2015)

Synthetic MRI in the Detection of Multiple Sclerosis Plaques

In this retrospective study, synthetic T2-weighted, FLAIR, double inversion recovery, and phase-sensitive inversion recovery images were produced in 12 patients with MS after quantification of T1 and T2 values and proton density. Double inversion recovery images were optimized for each patient by adjusting the TI. The number of visible plaques was determined by a radiologist for a set of these 4 types of synthetic MR images and a set of conventional T1-weighted inversion recovery, T2-weighted, and FLAIR images. Conventional 3D double inversion recovery and other available images were used as the criterion standard. Synthetic MR imaging enabled detection of more MS plaques than conventional MR imaging in a comparable acquisition time (approximately 7 minutes). The contrast for MS plaques on synthetic double inversion recovery images was better than on conventional double inversion recovery images. BACKGROUND AND PURPOSE: Synthetic MR imaging enables the creation of various contrast-weighted images including double inversion recovery and phase-sensitive inversion recovery from a single MR imaging quantification scan. Here, we assessed whether synthetic MR imaging is suitable for detecting MS plaques. MATERIALS AND METHODS: Quantitative and conventional MR imaging data on 12 patients with MS were retrospectively analyzed. Synthetic T2-weighted, FLAIR, double inversion recovery, and phase-sensitive inversion recovery images were produced after quantification of T1 and T2 values and proton density. Double inversion recovery images were optimized for each patient by adjusting the TI. The number of visible plaques was determined by a radiologist for a set of these 4 types of synthetic MR images and a set of conventional T1-weighted inversion recovery, T2-weighted, and FLAIR images. Conventional 3D double inversion recovery and other available images were used as the criterion standard. The total acquisition time of synthetic MR imaging was 7 minutes 12 seconds and that of conventional MR imaging was 6 minutes 29 seconds The lesion-to-WM contrast and lesion-to-WM contrast-to-noise ratio were calculated and compared between synthetic and conventional double inversion recovery images. RESULTS: The total plaques detected by synthetic and conventional MR images were 157 and 139, respectively (P = .014). The lesion-to-WM contrast and contrast-to-noise ratio on synthetic double inversion recovery images were superior to those on conventional double inversion recovery images (P = .001 and < 0.001, respectively). CONCLUSIONS: Synthetic MR imaging enabled detection of more MS plaques than conventional MR imaging in a comparable acquisition time. The contrast for MS plaques on synthetic double inversion recovery images was better than on conventional double inversion recovery images.

Utility of a Multiparametric Quantitative MRI Model That Assesses Myelin and Edema for Evaluating Plaques, Periplaque White Matter, and Normal-Appearing White Matter in Patients with Multiple Sclerosis: A Feasibility Study

BACKGROUND AND PURPOSE: T1 and T2 values and proton density can now be quantified on the basis of a single MR acquisition. The myelin and edema in a voxel can also be estimated from these values. The purpose of this study was to evaluate a multiparametric quantitative MR imaging model that assesses myelin and edema for characterizing plaques, periplaque white matter, and normal-appearing white matter in patients with MS. MATERIALS AND METHODS: We examined 3T quantitative MR imaging data from 21 patients with MS. The myelin partial volume, excess parenchymal water partial volume, the inverse of T1 and transverse T2 relaxation times (R1, R2), and proton density were compared among plaques, periplaque white matter, and normal-appearing white matter. RESULTS: All metrics differed significantly across the 3 groups (P < .001). Those in plaques differed most from those in normal-appearing white matter. The percentage changes of the metrics in plaques and periplaque white matter relative to normal-appearing white matter were significantly more different from zero for myelin partial volume (mean, −61.59 ± 20.28% [plaque relative to normal-appearing white matter], and mean, −10.51 ± 11.41% [periplaque white matter relative to normal-appearing white matter]), and excess parenchymal water partial volume (13.82 × 103 ± 49.47 × 103% and 51.33 × 102 ± 155.31 × 102%) than for R1 (−35.23 ± 13.93% and −6.08 ± 8.66%), R2 (−21.06 ± 11.39% and −4.79 ± 6.79%), and proton density (23.37 ± 10.30% and 3.37 ± 4.24%). CONCLUSIONS: Multiparametric quantitative MR imaging captures white matter damage in MS. Myelin partial volume and excess parenchymal water partial volume are more sensitive to the MS disease process than R1, R2, and proton density.

Contrast-enhanced synthetic MRI for the detection of brain metastases

Background Synthetic magnetic resonance imaging (MRI), a technique that enables creation of various contrast-weighted images from a single MRI quantification scan, is a useful clinical tool. However, there are currently no reports examining the use of contrast-enhanced synthetic MRI for detecting brain metastases. Purpose To assess whether contrast-enhanced synthetic MRI is suitable for detecting brain metastases. Material and Methods Ten patients with a combined total of 167 brain metastases who underwent quantitative MRI and conventional T1-weighted inversion recovery fast spin-echo (conventional T1IR) MRI before and after administration of a contrast agent were included in the study. Synthetic T1IR and T1-weighted (synthetic T1W) images were produced after parameter quantification. Lesion-to-white matter contrast and contrast-to-noise ratio were calculated for each image. The number of visible lesions in each image was determined by two neuroradiologists. Results The mean lesion-to-white matter contrast and mean contrast-to-noise ratio of the synthetic T1IR images were significantly higher than those of the synthetic T1W (P < 0.001 and P < 0.001, respectively) and conventional T1IR (P = 0.04 and P = 0.002, respectively) images. Totals of 130 and 124 metastases were detected in the synthetic T1IR images by the first and second radiologists, respectively. The corresponding numbers were 91 and 85 in the synthetic T1W images and 119 and 119 in the conventional T1IR images. Statistical significance was not found among detected numbers of lesions. Conclusion Synthetic T1IR imaging created better contrast compared with synthetic T1W or conventional T1IR imaging. The ability to detect brain metastases was comparable among these imaging.

Synthetic MR Imaging in the Diagnosis of Bacterial Meningitis

1Department of Radiology, Juntendo University School of Medicine, 2-1-1 Hongo, Bunkyo-ku, Tokyo 113-8421, Japan 2Department of Radiology, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan 3Department of Radiological Sciences, Graduate School of Human Health Sciences, Tokyo Metropolitan University, Tokyo, Japan 4Department of Pediatrics, Juntendo University School of Medicine, Tokyo, Japan

Automated brain tissue and myelin volumetry based on quantitative MR imaging with various in-plane resolutions

BACKGROUND AND PURPOSE Segmented brain tissue and myelin volumes can now be automatically calculated using dedicated software (SyMRI), which is based on quantification of R1 and R2 relaxation rates and proton density. The aim of this study was to determine the validity of SyMRI brain tissue and myelin volumetry using various in-plane resolutions. METHODS We scanned 10 healthy subjects on a 1.5T MR scanner with in-plane resolutions of 0.8, 2.0 and 3.0mm. Two scans were performed for each resolution. The acquisition time was 7-min and 24-sec for 0.8mm, 3-min and 9-sec for 2.0mm and 1-min and 56-sec for 3.0mm resolutions. The volumes of white matter (WM), gray matter (GM), cerebrospinal fluid (CSF), non-WM/GM/CSF (NoN), brain parenchymal volume (BPV), intracranial volume (ICV) and myelin were compared between in-plane resolutions. Repeatability for each resolution was then analyzed. RESULTS No significant differences in volumes measured were found between the different in-plane resolutions, except for NoN between 0.8mm and 2.0mm and between 2.0mm and 3.0mm. The repeatability error value for the WM, GM, CSF, NoN, BPV and myelin volumes relative to ICV was 0.97%, 1.01%, 0.65%, 0.86%, 1.06% and 0.25% in 0.8mm; 1.22%, 1.36%, 0.73%, 0.37%, 1.18% and 0.35% in 2.0mm and 1.18%, 1.02%, 0.96%, 0.45%, 1.36%, and 0.28% in 3.0mm resolutions. CONCLUSION SyMRI brain tissue and myelin volumetry with low in-plane resolution and short acquisition times is robust and has a good repeatability so could be useful for follow-up studies.

Analysis of White Matter Damage in Patients with Multiple Sclerosis via a Novel In Vivo MR Method for Measuring Myelin, Axons, and G-Ratio

BACKGROUND AND PURPOSE: Myelin and axon volume fractions can now be estimated via MR imaging in vivo, as can the g-ratio, which equals the ratio of the inner to the outer diameter of a nerve fiber. The purpose of this study was to evaluate WM damage in patients with MS via this novel MR imaging technique. MATERIALS AND METHODS: Twenty patients with relapsing-remitting MS with a combined total of 149 chronic plaques were analyzed. Myelin volume fraction was calculated based on simultaneous tissue relaxometry. Intracellular and CSF compartment volume fractions were quantified via neurite orientation dispersion and density imaging. Axon volume fraction and g-ratio were calculated by combining these measurements. Myelin and axon volume fractions and g-ratio were measured in plaques, periplaque WM, and normal-appearing WM. RESULTS: All metrics differed significantly across the 3 groups (P < .001, except P = .027 for g-ratio between periplaque WM and normal-appearing WM). Those in plaques differed most from those in normal-appearing WM. The percentage changes in plaque and periplaque WM metrics relative to normal-appearing WM were significantly larger in absolute value for myelin volume fraction than for axon volume fraction and g-ratio (P < .001, except P = .033 in periplaque WM relative to normal-appearing WM for comparison between myelin and axon volume fraction). CONCLUSIONS: In this in vivo MR imaging study, the myelin of WM was more damaged than axons in plaques and periplaque WM of patients with MS. Myelin and axon volume fractions and g-ratio may potentially be useful for evaluating WM damage in patients with MS.

Synthetic MRI showed increased myelin partial volume in the white matter of a patient with Sturge-Weber syndrome

Dear Editor-in-Chief, We read, with much interest, the article by Kim et al. [1] regarding myelin quantification in pediatric populations using SyMRI. SyMRI refers to the whole package of a specific relaxometry method and synthesis of contrast-weighted images (Bsynthetic MRI^) with measurement of myelin based on the acquired quantitative values [2]. They showed that the myelin volumes of 22 subjects, measured using SyMRI, were well fitted to the Gompertz model, where growth is the slowest at the start and end. Their study showed the potential utility of SyMRI myelin imaging. Because we have also studied the use of SyMRI in pediatric patients [3], especially those with Sturge-Weber syndrome [4, 5], we read this article with great personal interest. During a 2017 American Society of Neuroradiology Annual Meeting presentation entitled, BSynthetic MRI in Children: Clinical Utility of Quantitative Segmentation,^West et al. also reported the trajectory pattern of myelin volume changes in 620 children, showing results similar to those by Kim et al. [1]. We thank Kim et al. for referring to our article [4]. The authors mentioned that we showed abnormal myelination in one hemisphere of a patient with Sturge-Weber syndrome using a SyMRI myelin partial volume map. However, in our article, we did not mention myelin partial volume maps because the SyMRI software (version 7.2, SyntheticMR, Linköping, Sweden) that we used did not have the functionality for showing myelin partial volume maps. We reported that all of the T1 and T2 values, and the proton density, decreased in the cerebral white matter of the affected side and concluded that this observation may indicate so-called accelerated myelination, which had been suggested to occur in patients with Sturge-Weber syndrome. Because the current version of the SyMRI software (ver. 8.0) has the ability to show myelin partial volume maps, we re-analyzed the raw data and we would like to show the myelin partial volume map (Fig. 1) of the patient reported in our article [4]. The image shows that the myelin partial volume was grossly increased in the cerebral white matter of the right side (affected side) compared with the left hemisphere. The total volume of myelin was 4.83 ml in the right cerebral hemisphere and 2.06 ml in the left hemisphere. The percentage of the myelin volume within the brain parenchyma was 1.88% in the right cerebral hemisphere and 0.69% in the left hemisphere. The models investigated by Kim et al. [1] may enable us to estimate the myelination stage of each cerebral hemisphere separately in patients with Sturge-Weber syndrome. Even though this case does not have histopathology to prove the myelination status and it may be too early to conclude that Baccelerated myelination^ caused this * Akifumi Hagiwara a-hagiwara@juntendo.ac.jp

Spatial Restriction within Intracranial Epidermoid Cysts Observed Using Short Diffusion-time Diffusion-weighted Imaging

We report two cases of pathologically proven intracranial epidermoid cysts. Both cases were scanned with diffusion-weighted imaging using pulsed gradient spin-echo (PGSE) and oscillating gradient spin-echo (OGSE; 50 Hz) prototype sequences with diffusion times of 47.3 ms and 8.5 ms, respectively. The apparent diffusion coefficient measured by OGSE was higher than that measured by PGSE, indicating the spatial restriction of water diffusion in the laminated keratin layers within the cyst as demonstrated by histopathology.